When burning fossil fuels to produce energy, one typically uses a high temperature combustion process in the presence of air. Unfortunately, this type of process produces both nitrogen oxides (NOx), which are well-known pollutants, and other components that are harmful to health or the environment, such as carbon monoxide, sulfur oxides (SOx), volatile heavy metals, and unburned hydrocarbons. Thus, it is important to remove these materials prior to their release into the environment.
There have been many investigations into methods that allow for the removal of these substances. Combustion modifications and adsorption techniques can be used for this removal, but may suffer from limited maximum removal of pollutants such as NOx and limited capacity. A method for addressing the problem of noxious exhaust gases is catalytic removal, which by comparison, is extremely effective in removing large proportions of unwanted exhaust components and is capable of treating very large volumes of exit gases for long periods of time.
Selective catalytic reduction (SCR) has been one of the most effective technologies for removing NO (the main nitrogen oxide formed at high temperature) from a waste gas effluent originating from a stationary or mobile combustion source. Stationary combustion sources are mainly utility boilers, industrial boilers, incinerators, and cogeneration turbines. Mobile combustion sources are mainly vehicles such as automobiles, trucks.
The SCR process is widely used for example in the U.S., Japan, and Europe to reduce emissions of large utility boilers and other commercial applications. Increasingly, SCR processes are being used to reduce emissions in mobile applications such as in large diesel engines like those found on ships, diesel locomotives, automobiles and the like.
In order to effect the reduction of NOx in waste combustion gases through catalytic reduction processes, it is necessary either to introduce a reducing agent, such as ammonia, and/or to use the unburned hydrocarbons present in the waste gas effluent. The SCR process generally provides the reduction of NOx, (NO, N2O and NO2) species using the reducing agent (e.g., ammonia) in the presence of oxygen and a catalyst to produce molecular nitrogen and water.
Selective catalytic reduction (SCR) of NO by NH3 as reducing agent in the presence of O2, that causes the formation of NO2 (from the parent NO) in the gas reagent mixture, is a complex reaction can be schematized with the following series of main reactions (I) and (II):4NO+4NH3+O2→4N2+6H2O  (I)2NO2+4NH3+O2→3N2+6H2O  (II)Other common reactions that could be taken into account when low 02 concentration is present in the gas mixture are the following reactions (III) and (IV):NO+NO2+2NH3→2N2+3H2O  (III)6NO2+8NH3→7N2+12H2O  (IV).
At temperatures around 300-350° C., competition between the reaction of NO reduction by NH3 and the reaction of NH3 oxidation by oxygen forming NO and/or N2 and/or N2O can occur, according to the following reactions (V) to (VII):4NH3+5O2→4NO+6H2O  (V)2NH3+2O2→N2O+3H2O  (VI)4NH3+3O2→2N2+6H2O  (VII).
These last reactions (V) to (VII) sequestrate NH3, the reducing agent, to the SCR process: they are in competition with the previous reactions of NOx reduction. The occurrence of these reactions (V) to (VII) is responsible for the observation of a maximum of the conversion curve of the NOx species as a function of reaction temperature; that is to say, the rate of NOx (NO+NO2) reduction is not continuously increasing with temperature because, staring from a defined temperature, the concentration of the reducing agent (NH3) decreases due to its oxidation by oxygen.
The SCR process is a competitive reaction scheme constituted by series of parallel reactions, the kinetics of each reaction and its dependence on temperature has to be exploited in order to selectively obtain NO reduction by ammonia and to avoid ammonia oxidation by oxygen. Finding a catalytic system to selectively obtain NO reduction by ammonia in a large temperature interval is thus challenging.
Various catalysts have been used in the SCR processes. Catalysts, including various metals, transition metal oxides, and mixed metal oxides have been employed for NO reduction. Initial catalysts, which employed platinum or platinum group metals, were found unsatisfactory because of the need to operate in a temperature range in which explosive ammonium nitrate forms. In response to environmental regulations in Japan, the first vanadium/titanium SCR catalyst was developed, which has proven to be highly successful. Further development has resulted in the development of vanadium catalyst deposited on titanium oxide/tungsten oxide support material.
Metal-promoted zeolite catalysts including, among others, iron-promoted and copper-promoted zeolite catalysts, for the selective catalytic reduction of nitrogen oxides with ammonia are known.
Zeolites are aluminosilicate crystalline materials having rather uniform pore sizes which, depending upon the type of zeolite and the type and amount of cations included in the zeolite lattice, range from about 3 to 10 Angstroms in diameter.
Chabazite (CHA) is a small pore zeolite with 8 member-ring pore openings (˜3.8 Angstroms) accessible through its 3-dimensional porosity. A cage like structure results from the connection of double six-ring building units by 4 rings. WO 2008/106519 discloses a catalyst comprising: a zeolite having the CHA crystal structure and a mole ratio of silica to alumina greater than 15 and an atomic ratio of copper to aluminum exceeding 0.25. The catalyst is prepared via copper exchanging NH4+-form CHA with copper sulfate or copper acetate. The copper concentration of the aqueous copper sulfate ion-exchange step varies from 0.025 to 1 molar, where multiple copper ion-exchange steps are needed to attain target copper loadings. U.S. Pat. No. 8,293,199 describes processes for the preparation of copper containing molecular sieves with the CHA structure having a silica to alumina mole ratio greater than about 10, wherein the copper exchange step is conducted via wet state exchange and prior to the coating step and wherein in the copper exchange step a liquid copper solution is used wherein the concentration of copper is in the range of about 0.001 to about 0.25 molar using copper acetate and/or an ammoniacal solution of copper ions as copper source.
Acidic zeolites such as copper ion-exchanged Y zeolites, H mordenite, and Cu—H mordenite have been reported to be efficient catalysts for NO reduction by NH3, particularly for the high-temperature application of SCR technology—see for example, Choi et al, Journal of Catalysis (1996) vol. 161, pages 597-604; Putluru et al, Applied Catal. B: Environmental (2011) vol. 101, pages 183-188.
Iron-promoted zeolite beta (U.S. Pat. No. 4,961,917) has been an effective commercial catalyst for the selective reduction of nitrogen oxides with ammonia.
Unfortunately, it has been found that under harsh hydrothermal conditions, for example exhibited during regeneration of a soot filter with temperatures locally exceeding 700° C., the activity of many metal-promoted zeolites begins to decline. This decline is often attributed to dealumination of the zeolite and the consequent loss of metal-containing active centers within the zeolite.
The catalysts employed in the SCR process ideally should be able to retain good catalytic activity over the wide range of temperature conditions of use, for example, from 200° C. to 600° C. or higher, preferably under hydrothermal conditions since water is generated during NOx reduction (see reactions (I) and (II) above). Hydrothermal conditions are also encountered in practice, such as during the regeneration of a soot filter, a component of the exhaust gas treatment system used for the removal of particles.
Despite the various catalysts being developed such as the zeolite supported catalysts, there exists a need for lower cost catalysts that provide and maintain high catalytic activity in the SCR reaction, especially for once-through applications in which the catalyst is injected into a waste gas stream and where the contact time between the supported catalyst and NOx may be short in the order of a second to a few minutes.